The present invention relates to an apparatus for slitting a traveling web of paper, solid fiber paperboard, corrugated or composite paperboard, or webs made of other materials, such as plastics (including foam) and, in particular, to an air bearing assembly for supporting the running web below and in cooperation with an upper rotary slitting blade.
Various types of apparatus for longitudinally slitting a continuous running web of corrugated paperboard are known in the art. Such apparatus often includes a related mechanism for simultaneously providing longitudinal score lines in the advancing web, which score lines facilitate subsequent folding in the construction of boxes or the like. Thus, a combined slitter-scorer utilizes pairs of rotatable cutting tools and scoring tools disposed in the path of the advancing web of corrugated paperboard, with one tool of each pair disposed on an opposite side of the web. Typically, multiple slitting tools are mounted coaxially and laterally spaced across the width of the web and, likewise, multiple scoring tools are also coaxially mounted and spaced across the width of the web.
A conventional prior art device is shown for example in U.S. Pat. No. 4,627,214. Each pair of upper and lower slitting tools is disposed with overlapping radial cutting edges between which the advancing paperboard web is moved to provide a continuous slit. Although commonly referred to as a "slitting" operation, the cutting tools of this type of prior art device in fact shear the moving sheet, resulting in a relative vertical displacement of the adjacent cut edges. However, problems arise as the thickness of the web increases, resulting in cuts which tend to become more ragged, edges which tend to be crushed, and a general degradation in the slit quality. Prior art methods also generate significant amounts of dust, resulting in a wide variety of well known environmental, operational, maintenance and quality control problems.
U.S. Pat. No. 5,090,281 discloses a slitting apparatus which represents a significant improvement in the above described prior art shear-type slitting apparatus. In this apparatus, the paperboard web is cut with a true slitting technique utilizing an apparatus in which the advancing web is directed through a thin circular blade rotating at high speed and running in the same direction as the web, with the board supported below the blade by rollers in contact with the underside of the web. This apparatus reduces significantly the generation of paperboard dust in the slitting operation and improves slit quality.
U.S. Pat. No. 5,197,366 is directed to improvements in the supporting roller assembly for a slitting apparatus of the type described in U.S. Pat. No. 5,090,281. The improvements minimize the build up of adhesive on the supporting rollers and the entry of adhesive, board dust and board scraps into the blade-receiving slot in the supporting rollers. The assembly also facilitates the discharge of board scrap which does enter the slot.
Although slotted supporting anvil rollers have worked quite satisfactorily and provide much improved slit quality, particularly when slitting high quality corrugated paperboard, problems with poor slit quality still occur regardless of the kind of slitting apparatus used when the quality of the paper web diminishes. With the presently increasing trend toward the use of recycled papers, the quality of paper webs in many applications has deteriorated and it has been found that these webs are more difficult to slit without producing ragged slit edges or downturned edge portions, even when utilizing the true rotary slitting apparatus described in the above identified patents. For example, recycled paper is used increasingly for one or both liners and/or the medium in the manufacture of corrugated paperboard, and solid fiber board is also increasingly being made from recycled paper stock. The quality of products made from such inherently poorer quality papers could be enhanced with better slit quality. Improving the slit quality in poorer grades of paper web would inherently also inure to the benefit of higher quality paper webs, including plain paper, solid fiber board, or corrugated paperboard.